Participants met with a dietician twice a month to help them stay on track and researchers assessed participants' feelings of depression, anxiety, anger, and fatigue before the diet began, after eight weeks of dieting, 24 weeks, and at the end of the year.

Though participants consumed the same amount of calories and lost the same amount of weight -- 30 pounds on average -- only those on the low-fat diet maintained an increase in positive mood throughout the year.

"I'm not surprised at all that [dieters] would have a better mood while [still] eating healthy carbs," says Dr. Keith Ayoob, nutritionist and associate professor at Albert Einstein College of Medicine in New York. "When a diet is [very] low in carbs, it can start to wear you down."

The Carb Question

Ayoob adds that forcing your body to burn fat for energy instead of the carbohydrates it's used to, as dieters do during a low-carbohydrate diet, "is basically a starvation adaptation -- the way your body evolved to deal with long periods of calorie deprivation. It's a survival mechanism, not a way to live your daily life."

But many low-carb dieters would disagree. Amy Dungan, a blogger on low-carb dieting, has followed a low-carbohydrate diet similar to the one employed in the study for the better part of a decade and she credits it with helping her deal with her depression: "I was surprised by how it affected my mood, after the first few weeks I felt really good, not just emotionally, but my energy surged."

Dungan says that "it was the opposite with low fat diets", which she had tried with no success for years before switching to a low-carb regimen.

So why did mood fall for low-carb dieters in the study?

The authors said that this may be because low-carb diets have less flexibility than low-fat diets and can be more difficult to keep up.

Dr. Carl Lavie, Medical Director of Cardiac Rehabilitation and Prevention at the Ochsner Heart and Vascular Institute, agrees, saying that in his experience, dieters "have a very hard time sticking with a low-carbohydrate diet long-term and…become irritable on such a diet. A low fat-diet, on the other hand, is easier to tolerate long-term and does not seem to produce such irritability."